Groundwater is often contaminated with undesirable compounds. The contamination typically is the result of waste leaking or leaching into below-ground aquifers. Although many contaminants in water can be removed by passing the water through granular activated carbon, groundwater is generally not treated in this way, because known methods require the groundwater to be pumped above ground level. In many locations, regulations prohibit groundwater that is removed from the aquifer from being re-injected into the aquifer. Thus, water pumped to the surface is lost for groundwater use and becomes a disposal problem.
In situ methods of treating groundwater do not move the water from the aquifer above ground level. The groundwater is treated below ground level, and once treated, remains in the aquifer. One in situ approach that has been suggested involves filling the annular space between the screened portion of a well casing and the well's borehole with granular activated carbon. In this way, the groundwater could flow through the carbon when entering or exiting the well. However, this approach may be impractical. The carbon would effectively be acting as a gravel pack, replacing sand that would normally be placed between the screen and the borehole. This limits the composition of carbon that could be used. Furthermore, once exhausted, the carbon could not easily be replaced with charged carbon. If the carbon was removed, the borehole would almost certainly collapse, making it impractical to place fresh carbon around the well.
Another in situ method of treating groundwater involves using air to strip contaminants from the water. The water is not removed from the aquifer; only the air stream with the contaminants is brought to the surface. The air stream may then be treated to remove the contaminants stripped from the groundwater. Once the air is treated, it may be re-used to remove more contaminants from the groundwater, or released to the atmosphere. Alternatively, the air stream may be released to the atmosphere without treatment. Injection of air may disturb the chemical equilibrium in the groundwater, thus fouling the well with various precipitates and/or biological growths, including iron, manganese, calciferous solids, and iron-fouling bacteria.